The invention relates to a process for the preparation of 2,3-dichloro-nitrobenzene by nitration of 1,2-dichloro-benzene using an anhydrous mixture of nitric acid, sulphuric acid and phosphoric acid as the nitrating medium.
Dichloro-nitrobenzenes are important intermediates for the synthesis of pharmaceuticals and plant protection agents. The currently known processes for the nitration of 1,2-dichlorobenzene give a mixture of 2,3-dichloro-1-nitrobenzene and 3,4-dichloro-1-nitrobenzene. The market demand for the two compounds is subject to changes. Thus, there was a greater need hitherto for the 3,4-isomer. Under the nitrating conditions practised hitherto the formation of this isomer was favoured so that production could follow the market demand.
In the meantime however, the market demand for the 2,3-isomer has increased. In order to be able to satisfy this increased demand, without simultaneously producing additional 3,4-isomer, a production procedure is necessary with which the formation of the two isomers can be controlled, and with which in particular the production of the 2,3-isomer, which has hitherto been obtained in a great deficiency, can be increased compared with the 3,4-isomer.
The use of phosphoric acid in the nitrating acid mixture is already known from the nitration of monochlorobenzene, in order to influence the ratio of the two isomers, namely of 2-chloro- and of 4-chloro-nitrobenzene.
Thus, DE-OS (German Published Specification ) 2,422,305 describes a process in which the reaction is carried out using 90% strength nitric acid as the nitrating agent and in a nitrating medium consisting of phosphoric acid and sulphuric acid. Using this process, the ratio of the para to the ortho isomer can be shifted from 1.63 to 1.2. The said DE-OS ( German Published Specification ) '305 does not indicate a concentration of the phosphoric acid; it may therefore be suspected that commercially available, 85% strength by weight phosphoric acid is used. In addition, the said DE-OS (German Published Specification) '305 does not give any yields and any data for dinitration. According to our own investigations, the yields according to this process are only about 80% of the theoretical yield.
Yield data (up to 93%) are found in DE-OS (German Published Specification) 2,422,306, which is obviously an improvement in the process of DE-OS (German Published Specification) '305. To this end, a metal catalyst, for example one composed of molybdenum, manganese, vanadium or tungsten, is added to the acidic nitrating medium.
However, such a process brings serious disposal problems with it.
Both specifications, namely DE-OS (German Published Specification) '305 and DE-OS (German Published Specification) '306 contain no data whatsoever about a possible transfer of the processes described therein to dichlorobenzenes, especially to 1,2-dichlorobenzene.
Another process, which has been published in DE-OS (German Published Specification) 3,244,293, is also directed to the nitration of monochlorobenzene. In this, nitration is carried out in a nitrating medium which consists only of phosphoric acid, with at most one mol of nitric acid per mole of chlorobenzene. The phosphoric acid employed is employed in concentrated form with a P.sub.2 O.sub.5 content of more than 72.4% by weight. Sulphuric acid is completely dispensed with. According to the description of this DE-OS (German Published Specification) '293, a para/ortho ratio of the chloronitrobenzenes of 0.9 can be obtained. The yields, relative to the nitric acid employed, are very high if the nitric acid is employed in a relatively large deficit. However, is simultaneously means incomplete reaction of the chlorobenzene. Such an inadequate conversion is only justifiable with economical starting materials, such as monochlorobenzene, and moreover presents recycling problems. This DE-OS (German Published Specification) '293 also gives no indication at all of whether the process described therein can be transferred to other substrates, such as dichlorobenzenes, especially to 1,2-dichlorobenzene.
The restriction of the process descriptions mentioned to monochlorobenzene is therefore understandable, in that dichlorobenzenes are basically slow to react owing to the deactivating effect of the second halogen atom, the competing directing effect of two substituents being added as an additional difficulty. The need therefore existed for a process for the preparation of 2,3-dichloro-nitrobenzene, which gives high yields and at the same time offers the possibility of satisfying an increased demand for 2,3-dichloro-nitrobenzene.